Such a method is known from U.S. Pat. No. 6,353,225.
In spectroscopic gas detection, the concentration of a known gas component in a gas mixture (measuring gas) is determined from a measured wavelength-specific absorption of the gas component. For this purpose, the measuring gas is introduced in a measuring volume having a predetermined optical path length, e.g. a sample cell or, in case of in-situ process measurements, a gas-leading pipe, furnace, funnel or the like. The light of a tunable light source, such as a diode laser, is transmitted through the measuring volume to a detector for generating a signal dependent on the wavelength-selective light absorption in the optical path of the measuring volume.
The gas mixture in which the measurement is performed is generally not well known and the gas component of interest must be measured with minimum interference from this background. It is therefore important to maintain an accurate lock to the absorption line of said gas component. This can be done by incorporating a reference cell which encloses the gas component of interest or another suitable gas component of constant concentration. This cell can be placed in an optical reference path which is split off from the main optical path through the measuring volume or it can be placed in the main optical path in-line with the measuring volume.
U.S. Pat. Nos. 4,410,273, 5,026,991, 5,173,749 and 5,459,574 show several variants of an absorption spectroscopy system, all using a beam splitter or an optical fiber coupler. The beam splitter and coupler invariably contribute to the noise of the system due to etalon effects. U.S. Pat. No. 5,459,574 further discloses an off-line locked spectroscopy system wherein the reference cell contains a reference gas having an absorption wavelength differing from that of the gas component of interest in the measuring volume by a predetermined amount. The purpose of this is to solve the problem that certain gases cannot be contained in a reference cell since they will react with or corrode the cell. The wavelength of the light source is locked to the absorption wavelength of the reference gas and then by means of a controller displaced by said predetermined amount to the absorption wavelength of the gas component to be measured. The precision of this offset is, however, depending on the precision of the electronic controller and can be subject to instabilities and temperature drift. Moreover, the light can accidentally be offset so that the optical emission wavelength coincides with that of an interfering gas in the gas mixture in the measurement volume.
The initially mentioned U.S. Pat. No. 6,353,225 shows an absorption spectroscopy system comprising a reference cell placed in the main optical path in-line with the measuring volume. The reference cell contains a sufficient amount of the gas component to be measured so that permanent preabsorption takes place. The gas concentration to be measured is obtained from the growth of the absorption. Alternatively to the reference gas containing a portion of the gas component to be measured, a neighboring atmospheric line of H2O or CO2, for example, can be used as a wavelength reference; however, if measurements are performed in an unknown gas mixture it is quite likely that these gases will be found and that their absorption will add to that of the reference gas in an unknown way.
The intensity of the light impinging onto the detector depends on both the wavelength-specific absorption by the gases in the optical path and the wavelength-independent total optical transmission including optical losses in the measuring system and the measurement path. Thus, normalization of the signal of the detector is necessary.
The most straight forward method to measure the non-gas related transmission is to use a direct detection. The wavelength of the light is swept by a triangular or sawtooth waveform over the absorption line of the gas component to be measured. The peak of the received triangularly or sawtooth shaped optical signal, which is well separated from the absorption peak, is compared with the signal from a monitor detector which directly monitors the output intensity of the light source. [Applied Optics, Vol. 38, Issue 36, pp. 7342-7354 (December 1999) and Applied Optics, Vol. 44, Issue 1, pp. 91-102 (January 2005)]. In wavelength modulation spectroscopy (WMS) a combination of wavelength modulation and direct detection can be used [Applied Optics, Vol. 38, Issue 21, pp. 4609-4622 (July 1999)]. These techniques are mostly developed for atmospheric monitoring; in order to be used in harsh industrial environment, the modulation rate has to be increased in order to place the signal energy above that of the turbulent measuring medium.
In wavelength modulation spectroscopy (WMS) an indirect measure of the non-gas related optical transmission can be obtained by the use of the wavelength modulation signal [U.S. Pat. No. 5,173,749], which makes it necessary to introduce a separate detection chain for the fundamental frequency. An intentionally injected pilot tone at a higher harmonic of said wavelength modulation signal [U.S. Pat. No. 7,116,422] avoids the use of a separate electronic channel.